The invention relates to methods for applying protective coatings of a desired composition on articles by physical vapor deposition.
The thermal evaporation and condensation of solid materials, such as metals, ceramics and inorganic compounds, to form layers is a developed art. There are many prior art techniques and apparatuses that permit such materials to be evaporated from a source and condensed to form a layer on a substrate that is disposed a distance from the source. The processes involve heating a material to be evaporated to a temperature at which it has a significant vapor pressure, thus creating a vapor stream. Heating techniques include direct heating methods, such as heating the material to be deposited directly using resistance, induction, electron beam or lasers to melt all or some portion of the material to be evaporated, or indirect heating methods, such as by heating the surface of a higher melting material and flashing the material to be evaporated off the hot surface.
A coating composition is related to the composition of the evaporant source. Typically, substrates are positioned perpendicularly above, (i.e., in "line-of-sight"), of a vapor source for coating thereby. Different coating compositions at the line-of-sight position can be achieved by varying the composition of the evaporant source.
Generally, the evaporant source should be replenished as material is gradually consumed during the evaporation process. In this respect, where the vapor source is a molten pool held in a crucible, the bottom of the crucible can be adapted to provide an opening for receiving the continuous feed of a solid bar or ingot of the evaporating material. The evaporant source composition and, therefore, coating composition are dependent on the composition of the ingot. Accordingly, a desired coating composition at the line-of-sight position can be achieved by controlling ingot composition.
In some cases, depending on the materials involved, ingot composition is dictated by fabrication considerations. For instance, for a given group of materials, attempt to manufacture ingots of certain compositions will result in ingots that are unduly brittle or otherwise mechanically unsuitable. In such cases, control of ingot composition is severely limited and, therefore, control of coating composition at the line-of-sight is more difficult. Therefore, a need exists for a process to deposit materials for controlling the deposit composition.